Friends who have each other's backs, look out for each other, spend lots of time together, and are just really good friends.
They have inside jokes together, they go to the park and swing on swing sets, get ice cream, go to the beach, go to concerts, go bowling, and basically any activity that they like doing together as friends.

1. Kaely and Xander are besties!

2. Kaely: Yesterday, I went to the park and played on the swings with Xander!
Sam: Well of course! You're besties!

A slang term for best-friend...Goes hand in hand with bestest friend...
One who plays Yatzee with you...Drinks with you...Comes over just because your having a bad day...Or even blows off a date to hang out with you...Sometimes they watch "The Emperors New Grove" together...Have Fridays on Tuesdays night, and together graffiti a wall...